Getting Physical – Box2D - Cocos2d-x by Example Beginner's Guide - page 300

Time for action - adding the touch events
We'll deal with onTouchBegan first.
1. In the onTouchBegan method, we start by updating the game state:
bool GameLayer::onTouchBegan(Touch * touch, Event *
event) {
if (!_running) return true;
if (_gameState == kGameOver) {
if (_gameOver->isVisible())
_gameOver->setVisible(false);
resetGame();
return true;
}
2. Next, we check on the value of _canShoot . This returns true if the white ball
is not moving.
if (!_canShoot) return true;
3. Next, we determine whether the touch is landing on the white ball. If it is, we start
the game if it is not currently running yet and we make our timer visible. Here's the
code to do this:
if (touch) {
auto tap = touch->getLocation();
auto playerPos = _player->getPosition();
float diffx = tap.x - playerPos.x;
float diffy = tap.y - playerPos.y;
float diff = pow(diffx, 2) + pow(diffy, 2);
if (diff < pow(BALL_RADIUS * 4, 2)) {
if (_gameState != kGamePlay) {
_gameState = kGamePlay;
if (_intro->isVisible())
_intro->setVisible(false);